Using old code with new versions of PHP

Now that PHP has grown to be a popular scripting language, there are
a lot of public repositories and libraries containing code you can reuse.
The PHP developers have largely tried to preserve backwards compatibility,
so a script written for an older version will run (ideally) without changes
in a newer version of PHP. In practice, some changes will usually be needed.

External variables are no longer registered in the global scope by
default. In other words, as of PHP
» 4.2.0 the PHP directive
register_globals is
off by default in php.ini. The preferred
method of accessing these values is via the superglobal arrays mentioned
above. Older scripts, books, and tutorials may rely on this
directive being on. If it were on,
for example, one could use $id from the URL
http://www.example.com/foo.php?id=42. Whether on
or off, $_GET['id'] is available.